Parthian Empire

The Parthian Empire was an Iranian empire that flourished from 270 BC to 224 AD, when the Sassanian Empire conquered them. The Parthians were originally part of the Dahae but branched off and conquered Persia from the Seleucid Empire, forming their own independent empire that stretched from the Indus River Valley to the Mediterranean Sea.

Background
The Parthians were a people that hailed from the regions of Elymais and Medea in Persia and Tribus Sakae in Russia, a group of steppe horsemen. They evolved from the old steppe nomads of Iran and Dahae, and after Dahae was conquered by Alexander III of Macedon, the Parthians lived in obscurity until 270 BC, when they gained independence from the Seleucid Empire, founded by one of Alexander's Diadochi, Seleukos.

Expansion
The Parthians' first leader was Arsaces, who centered his armies around Cataphracts and horse archers. His natural foes were his immediate rivals: the Seleucid Empire, Artaxiad Empire, and Scythia. In 268 BC he defeated the Armenians at the Battle of Lasphia, followed by victories by his son and successor Ardumanish I of Parthia; he died of fever shortly after. The reign of Ardumanish was comparable to that of Sargon of Akkade, modernizing his armies and conquering almost all of the Middle East using mercenary infantry and horse-mounted archers. The Parthians later went to war with the Ptolemaic Empire when the Egyptians attacked them to halt their transgression.